(1) Field of the Invention
A process for the desulfurization of sulfur-containing hydrocarbon feedstocks, especially reformer feedstocks. In particular, it relates to a process for the removal of sulfur from hydrofined reformer feedstocks, without reduction in the temperature of the feedstock as received from the hydrofiner, via the use of an adsorbent, or sorbent.
(2) Background and Prior Art
It is known to use sorbents for the removal of sulfur from process streams, including particularly hydrocarbon process streams. The nature and quality of such sorbents varies widely, particularly as relates to this capacity to remove a wide variety of sulfur species, viz., mercaptans, thiophenes, disulfides, thioethers, hydrogen sulfide, carbonyl sulfide, and the like. Such sorbents, in particular, are not especially suitable for the essentially complete removal of sulfur from process streams, as required in some operations, e.g., catalytic reforming, or hydroforming, a well-known and important process employed in the petroleum refining industry for improving the octane quality of naphthas and straight run gasolines.
The presence of sulfur even in a small and virtually infinitesmal concentrations can have a detrimental effect in reforming. In a typical reforming process, a series of reactors of the reforming unit are each provided with fixed beds of sulfided catalyst which are sequentially contacted with a naphtha feed, and hydrogen at high severities, e.g., at high temperatures and low pressures. Each reactor is provided with a preheater, or interstage heater, because the reactions which take place are endothermic, and the temperature between the several reactors is progressively increased. In use of the more recently developed polymetallic platinum catalysts wherein an additional metal, or metals, hydrogenation-dehydrogenation (hydrogen transfer) component is added as a promoter to the platinum, it is, in fact, essential to reduce the feed sulfur to only a few parts, per million parts by weight of feed (wppm), because of the extreme sulfur sensitivity of these catalysts. For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed well below about 2 wppm, and preferably below about 0.5 wppm, or even less than about 0.1 wppm to avoid excessive loss of catalyst activity and C.sub.5.sup.+ liquid yield.
In most, if not all, commercial reforming operations, a sulfur-containing straight run gasoline, or naphtha, is first hydrofined (or hydrodesulfurized) to remove a preponderance of the sulfur, and the desulfurized feed than reformed. In the hydrofiner a sulfur-containing straight-run gasoline, or naphtha, is contacted with hydrogen, over a Group VIB and/or Group VIII metal catalyst, e.g., cobalt molybdate or nickel molybdate supported on alumina, at conditions sufficient to remove a preponderance of the sulfur as hydrogen sulfide, and the liquid product recovered for use as feed to the reformer. The efficiency of such units are limited by equilibrium or kinetic considerations and unfortunately the hydrofiner cannot reduce the sulfur levels to the amounts that are desired, or required; which may be on the order of 2 wppm, 0.5 wppm, or even 0.1 wppm, or less. Moreover, even if hydrofiners were capable of such effective operation, they are not capable of such operation 100 percent of the time. Upsets can and do occur. Typically, the naphtha feed will contain as much as 5 wppm to about 50 wppm, or more, of sulfur; and, if upsets occur, the feed during some portions of an operating cycle will contain even higher amounts of sulfur. These relatively high levels of feed sulfur will not only seriously adversely affect the level of C.sub.5.sup.+ liquid yield production and decrease the activity of the catalyst when sulfur-sensitive polymetallic platinum catalysts are used, but may also poison the catalyst such that regeneration of the catalyst may become necessary prior to its further, continued use.
Sorbent or catalyst packed guard chambers, or vessels filled with sorbents or catalysts, consequently have been used to remove additional sulfur from hydrofined products prior to their use as reformer feeds. Massive nickel catalysts, e.g., nickel on a silica-alumina support, have been particularly effective in removing sulfur from hydrofined products, or naphthas at temperatures ranging below about 350.degree. F. Higher temperatures than about 350.degree. F. cannot be used for the removal of sulfur from naphthas with massive nickel catalysts, however, because the production of fused multi-ring aromatics or polynuclear aromatic compounds (PNA's) in the naphtha becomes excessive. The presence of PNA's are undesirable, not only in that they cause deactivation of reformer catalysts, but they are also primarily responsible for the octane requirement increase (ORI) known to occur in automobile engines. In brief, ORI is caused by the build-up of carbonaceous deposits in internal combustion engines (particularly old engines) which, by limiting heat transfer from the combustion chamber, leads to preignition. Preignition causes the phenomenon known as engine knock or ping which is "cured" by burning higher octane gasolines. The PNA's are major contributors to the build-up of carbon deposits over the lifetime of an engine, which leads to ORI. Dependent on gasoline composition, the ORI can be as small as 1-2 octane numbers or as great as 10-12 octane numbers.
In many refineries, the naphtha feed from the hydrofiner is available at temperatures far in excess of 350.degree. F., e.g., 500.degree. F. and higher, and whereas expensive heat exchange processes might be employed to reduce the temperature to 350.degree. F., or less, the naphtha, after such treatment, would have to be reheated to the temperature required for reforming. This step is obviously quite burdensome, particularly in these times of increasing fuel costs. Hence, there is a particular need for a sorbent, or catalyst, useful for removing sulfur at temperatures above about 350.degree. F., or at temperatures ranging about above 350.degree. F. up to reforming temperatures; temperatures which normally produce excessive PNA's.